Electron beam gun assembly for producing a ribbon shaped beam and magnet means for transversely deflecting the beam about its major axis



mm, may;

R. W. FISK FIG. 2.

gum

DEFLECTING THE BEAM ABOUT ITS MAJOR AXIS Filed Aug. 11, 1967 .i U m i. Q CROSS REFERENCE May 26, 1970 ELECTRON BEAM GUN ASSEMBLY FOR PRODUCING A RIBBON SHAPED BEAM AND MAGNET MEANS FOR TRANSVERSELY DEFLECTION AND FOCUSING SYSTEM ROBERT W. FISK MW. PM,M

ATTORNEYS FIG. 3.

United States Patent 3,514,656 ELECTRON BEAM GUN ASSEMBLY FOR PRODUC= ING A RIBBON SHAPED BEAM AND MAGNET MEANS FOR TRANSVERSELY DEFLECTING THE BEAM ABOUT ITS MAJOR AXIS Robert W. Fisk, Sunnyvale, Calif., assignor to Air Reduction Company, Incorporated, New York, N.Y., a corporation of New York Filed Aug. 11, 1967, Ser. No. 660,024

Int. Cl. H01j 29/70 a US. Cl. 313-75 12 Claims ABSTRACT OF THE DISCLOSURE Apparatus is described which establishes a transverse magnetic field to deflect an electron beam from an initial path while causing convergence of the beam.

This invention relates to electron beam apparatus and, more particularly, to an improved electron beam gun assembly for deflecting an electron beam through a curved path, the gun assembly being particularly suited for use in an electron beam furnace.

Electron beam furnaces utilize one or more electron beam gun assemblies for producing high energy electron beams and directing such beams to a target to be heated. It has been found advantageous to use transverse magnetic fields (that is, those fields which have lines of flux extending transversely of the beam path) for deflecting the beam through a curved path. This permits the electron emitter to be placed in a location wherein it is less likely to be damaged by direct condensation of vapor or by ion bombardment.

Under some circumstances, it may be desirable to produce a convergence or focusing of an electron beam, to thereby reduce its cross sectional area, or at least to prevent enlargement thereof. For example, an electron beam of large cross sectional area may require excessive magnetic field sizes for deflecting it, and an inefficient transfer of energy to the target being heated may also result. Moreover, where it is desirable to pass the beam to the target area from a separate vacuum chamber, too large an opening between the chambers, which is normally required to pass a large cross section electron beam, can greatly increase contamination in the gun chamber.

Heretofore, many electron beam gun assemblies designed to achieve good convergence or forming of beams and deflection of the beams have been complex in construction. This is particularly true in connection with ribbon shaped electron beams, that is, those beams which are of elongated (ideally rectangular) cross section and which are produced by elongated cathodes or emitters. A gun assembly of complex construction may not only be expensive, but may occupy an inconvenient amount of space.

It is an object of this invention to provide an improved electron beam gun assembly.

Another object of the invention is to provide an electron beam gun assembly for producing a ribbon shaped electron beam and which achieves focusing of the beam after deflection of the beam of the order of 90.

A further object of the invention is to provide an electron beam gun assembly which produces a ribbon shaped electron beam and which deflects the beam approximately 90 while providing focusing thereof both laterally and longitudinally.

Still another object of the invention is to provide an electron beam gun assembly of the type described which aflords close control over the focusing and deflection of the electron beam.

3,514,656 Patented May 26, 1970 ice Other objects of the invention will become apparent to those skilled in the art from the following description taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic sectional view illustrating parts of an electron beam furnace system utilizing the invention;

FIG. 2 is a front elevational view of an electron beam gun assembly constructed in accordance with the invention;

FIG. 3 is a sectional view taken along the line 3-3 of FIG. 2;

FIG. 4 is a sectional view taken along the line 44 of FIG. 2;

FIG. 5 is a schematic cross-sectional view illustrating the cross section of a ribbon shaped electron beam at a certain distance from its source;

FIG. 6 is a schematic cross-sectional view illustrating the cross section of an electron beam after it has been focused and deflected in accordance with the invention, at the position indicated by the line 66 in FIG. 3; and

FIG. 7 is a schematic front elevational view illustrating an alternative embodiment of the invention.

Very generally, the electron beam gun assembly of the invention causes deflection and focusing of a ribbon shaped electron beam by utilizing a pair of elongated bar shaped pole pieces for establishing a transverse magnetic field in the initial path of the beam. The beam is deflected through a curved path as it passes through the transverse field. The fringes of the field are generally curved and form, with the region directly between the bar shaped pole pieces, a continuous magnetic field having curved outer limits. The beam is aimed in its initial path such that electrons toward the outer edge of the electron beam have a longer path length in the transverse field relative to those electrons toward the inner edge of the curving beam.

Referring now particularly to FIG. 1, an electron beam furnace system in which the invention is used is shown schematically. The system includes a vacuum enclosure having an outer wall 11, parts of which are illustrated. The enclosure is divided by a wall 12 into an upper chamber 13 and a lower chamber 14. Suitable vacuum pumps (not shown) may be provided for each of the two chambers. When evaporation is taking place in the chamber 13, separate evacuation of the chamber 14 helps to minimize the amount of vapor therein.

An electron beam gun assembly 16 is positioned in the low vapor environment of the chamber 14. The substantial absence of vapor in the chamber 14 facilitates the operation of the electron beam gun assembly. The electron beam gun assembly will be described in greater detail subsequently, and produces an electron beam 17 of a ribbon shape (that is, of elongated or ideally rectangular cross section), the extreme edges of which are indicated by dotted lines. The beam 17 is passed through an opening 18 in the wall 12 into the upper chamber 13.

The beam 17 is deflected through about 220 in the chamber 13 to impinge upon and melt target material 19 held in a crucible 21. The particular illustrated furnace system is for evaporating the target material, however, the invention is applicable to heating for other purposes as well. The crucible 21 is provided with coolant passages 22 through which a coolant is circulated for cooling the crucible. A skull 23 of the target material in solid form forms between the molten target material and the crucible walls. The material in the crucible may be replenished, either continuously or periodically, by a suitable stock material feed, not illustrated.

The deflection of the electron beam 17 in the chamber 13 is accomplished by a deflection and focusing system 24. This system may be of any suitable type, such as, for

example, a system generally in accordance with the teachings of copending application, Ser. No. 464,968, assigned to the assignee of the present invention and which is now US. Pat. No. 3,420,977 issued Jan. 7, 1969.

The particular electron beam furnace system shown in FIG. 1 is for purposes of illustration only and the invention, which will be described in detail subsequently, is not limited to use in such a system. The invention is, however, of particular advantage in such a system since it is possible to achieve a very small beam cross section at the opening 18, thereby minimizing the size of the opening required. By minimizing the size of the opening, fewer vapor particles will enter the chamber 14 from the chamber 13.

Referring now to FIGS. 2, 3 and 4, an electron beam gun assembly constructed in accordance with the invention is illustrated. The electron beam gun assembly 16 includes an elongated emitter 26 for producing electrons. The emitter 26 is preferably a tungsten wire, and extends between two supporting members 27 and 28. Means not illustrated provide a direct current potential across the members 27 and 28, resulting in a flow of direct current through the emitter 26. The current flow raises the temperature of the emitter causing it to produce free electrons.

The free electrons produced by the emitter 26 are reflected on three sides by a shaping electrode 29. The electrode 29 is insulated from the emitter support members 27 and 28 by insulating strips 31 and 32, respectively. The shaping electrode 29 is formed with an elongated recess 33 through which the emitter 26 extends. When the shaping electrode is maintained at the emitter potential, by suitable connection not illustrated, the electrons produced by the emitter 26 tend to move out of the open end of the recess 33 and away from the shaping electrode 29.

The electrons leaving the recess 33 in the shaping elec trode 29 are accelerated into a beam by an accelerating electrode 37 and pass through an opening 36 therein. The electrode 37 consists of a plate with two right-angle extensions 38 and 39 thereon which are attached to suitable mounting means, not illustrated. The plate 37 is maintained at a positive potential with respect to the emitter to produce an acceleration of the electrons. The result is a ribbon beam, that is, an electron beam having an elongated cross section which is ideally a narrow rectangle but which approximates a narrow oval. The beam has a major axis plane which extends through the emitter.

As may be seen in FIG. 3, the beam leaves the emit ter 26 at an acute angle in the major axis plane. In FIG. 3, the three dash-dot lines represent the opposite edges and the axis of the ribbon beam, and the angle designated as a is less than 90. The nonnormal orientation of the initial beam path with respect to the emitter is caused by the high intensity circumferential field produced by D-C heating current passing through the emitter.

After leaving the anode opening 36, the electron beam 17 is deflected about 90 through a curved path by means of a transverse magnetic field. The transverse magnetic field is established in the initial path of the beam between a pair of elongated bar shaped pole pieces 41 and 42, preferably of rectangular cross sections. The pole pieces extend generally parallel with the emitter 26 and each other, and are positioned on either side of the beam 17. A magnet 43 extends between the upper ends of the pole pieces 41 and 42, and a magnet 44 extends between the opposite ends of the pole pieces 41 and 42. The two magnets are identically oriented with regard to their polarities, being electromagnets connected to a control circuit and power supply 46. As viewed in FIG. 2, the pole piece 41 is polarized south and the pole piece 42 is polarized north, thereby causing an upward deflection of the electron beam 17 For purposes of this discussion, several reference planes will be referred to. A plane including the axes of both elongated pole pieces, indicated as the line A in FIG. 3 (a plane perpendicular to the plane of the paper), is referred to herein as the plane of the transverse field. The major axis plane of the beam is a plane in the plane of the paper in FIG. 3 and is referred to in FIG. 4 as the line B. The axis of the beam (a line) is indicated at C in FIG. 3 and lies in the plane B (FIG. 4). The initial straight segment of the axis line C in FIG. 3 adjacent the emitter 26 represents the initial path or injection direction of the beam before it enters the transverse field.

To obtain deflection and focussing in accordance with the invention, the beam is injected into the magnetic field defined by the pole pieces 41 and 42 so that the initial path of the beam intersects the plane of the transverse field (plane A) at an acute angle which lies in the major axis plane of the beam (plane B). This angle opens toward the direction in which the beam is deflected. With reference to the illustrated apparatus, this means that the straight portion of the axis C of the beam (the portion representing the injection direction) intersects the lane of the transverse field to form an angle 3 which lies in the major axis plane of the electron beam and which is equal to the angle a.

The fringe of the transverse field defined by the pole pieces 41 and 42 has a curving periphery as may be seen from the distribution of crosses-45 in FIG. 3 (representing the lines of flux). In addition, the flux lines themselves in this fringe region are curved as may be seen in FIG. 4. Thus, the field may be roughly analogized to a barrel in which the staves represent the lines of flux and which the top and bottom of the barrel are the pole pieces. The eifect of the curved lines of flux is discussed below. The effect of the curving periphery of the field, coupled with the beam injection angle, is to cause the electrons in the beam toward the lower edge thereof, as viewed in FIG. 3, to have a longer path length in the transverse magnetic field including its fringe than those electrons toward the opposite edge of the beam. Since there is a path length differential, and since those electrons subject to the influence of the field for a longer time will be deflected a correspondingly greater amount, the edges of the beam converge in the major axis plane as indicated in FIG. 3 (longitudinal focusing). This focusing action is of advantage for concentrating the cross section of the beam to effect a more efficient heating of a target surface or, as illustrated in FIG. 1, to pass through an opening of minimal size.

By using the fringe areas of the transverse magnetic field as described, with suitable emitter orientation, longitudinal focusing is achieved with very simple pole piece configuration. Small pole pieces can be of considerable advantage in high vacuum furnace work where space within the vacuum enclosure is minimal. Moreover, the simplicity of the described construction provides cost savings in materials and manufacture.

In order to facilitate precise adjustment of the focusing of the electron beam, the control and power supply circuit 46 is made such as to enable relative adjustment between the strengths of the electromagnets 43 and 44. Accordingly, a gradient in the strength of the magnetic field from one end of the pole pieces to the other may be established if desired. Where the gradient is made stronger toward the lower end of the pole pieces 41 and 42, as viewed in FIGS. 2 and 3, and is of decreasing strength in the direction toward which the beam is deflected (upwardly in FIGS. 2 and 3) a more sharp focusing action may be achieved. This is because the stronger the field through which electrons are passing, the smaller the radius of curvature of the electrons as they are deflected through a curving path. Thus, by suitably adjusting the relative strengths of the electromagnets 43 and 44, precise longitudinal focusing may be achieved.

Referring now to FIG. 4, the invention also provides for lateral focusing of the electron beam 17. It will be noted, as mentioned previously, that the flux lines in the fringe region of the field defined by the pole pieces 41 and 42, such flux lines being indicated by dotted lines in FIG. 4, are bowed. The action of forces acting in the bowed magnetic field causes the electrons in the beam to be deflected toward each other as indicated by the dash-dot lines in FIG. 4. Thus, a lateral focusing is accomplished in addition to the longitudinal focusing of the beam previously described. The strength of the magnetic field and the magnetic field gradient (if any) are selected so that the lateral focusing action and the transverse focusing action produce convergence in their respective directions at about the same point.

The result of the focusing action of the apparatus of the invention may be observed from a comparison of FIGS. 5 and 6. FIG. 5 illustrates a typical electron beam cross section in a situation where the beam is produced and directed from an arrangement similar to that illustr'ated in FIGS. 2, 3 and 4, only without a transverse magnetic field established by the pole pieces. The beam consists of a major central portion 47 and a pair of side portions 48 and 49. Such a beam pattern requires a relatively large opening in the event it is to be passed through a wall with low losses and effects a relatively inefficient transfer of energy as opposed to a more roundish beam cross section as indicated in FIG. 6. The cross section 51 indicated in FIG. 6 approximates the cross section of the beam taken at the line 66 in FIG. 3. It will be observed that the cross section thereat is roundish, minimizing the size of the opening 18 and, if used to directly impinge upon a target, effecting a more efficient heat transfer from the beam to the target material.

Referring now to FIG. 7, an alternative embodiment of the invention is shown schematically. Only one electromagnet 52 is shown extending between the lower ends of a pair of pole pieces 53 and 54. The elongated emitter 56, extending between emitter supporting members 57 and 58, produces a ribbon-shaped electron beam similar to that produced by the emitter 26. Suitable shaping and accelerating electrodes, not illustrated, may be provided.

In order to produce a gradient in the strength of the magnetic field between the pole pieces 53 and 54 where only one electromagnet 52 is used, the pole pieces are flared outwardly. The angle of flare depends upon the desired field gradient and may be selected to produce a desired deflection and focusing action.

It should be pointed out that the apparatus described herein utilizes a directly heated emitter. In such a case, because of the field set up by the heating current, it is desirable that the angle be less than 90 and this may be achieved by ,utilizing pole pieces which lie in a plane parallel with the emitter. In the case of an indirectly heated emitter where the electron beam leaves the emitter at a substantially normal path therefrom, it may be necessary to make the pole pieces not parallel with the emitter in order to achieve the desired acute angle of entry of the electron beam into the transverse magnetic field. Convergence may be controlled by variation of the magnetic field gradient from end to end of the pole pieces and by variation of the alignment of the emitter with the poles.

A desired ratio between the spacing of the pole pieces from each other and from the emitter is of the order of 4 or 5 to 3. Using an emitter of 5 inches length and a pair of parallel pole pieces of 8 inches length, /2 inch width and /2 inch thickness, satisfactory results may be achieved where the pole pieces are spaced a distance of inch from the emitter and a distance of 1% inches from each other, and where a D-C emitter current of 100 amps, a magnetic intensity between the pole pieces of 55 gausses, and a beam accelerating voltage of 10 kv. are used. The angle a may be about 11. Similarly, satisfactory results may be attained from the same structure where an emitter current of 100 amps, a magnetic intensity between the pole pieces of 90 gausses, and a beam accelerating volt- 6 age of 18 kv. are used. In this instance, the angle a may be about 8".

It may therefore be seen that the invention provides an improved apparatus for producing and directing an electron beam. The invention is of use in connection with a high vacuum electron beam furnace system, particularly where there are considerations of contamination of the apparatus which produces the electron beam as a re-= sult of vapor particle impingement, etc. The invention provides deflection of an electron beam through a desired angle, which may be approximately while producing focusing both laterally and longitudinally. Moreover, the invention provides accurate control over the focus and deflection of the electron beam to take into account varying conditions.

Various embodiments of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such other embodiments, and modifications thereof, are intended to fall within the scope of the appended claims.

What is claimed is:

1. An electron beam gun assembly for heating a target in a furnace comprising a pair of spaced, elongated bar shaped pole pieces for defining a magnetic field therebetween which extends parallel to a transverse plane which includes the axes of both pole pieces, and means disposed adjacent said pole pieces for producing a ribbon-shaped electron beam with its major axis plane extending transversely to said magnetic field and with its major axis directed in an. initially generally straight path into said magnetic field at an acute angle to said transverse plane, which angle lies in the major axis plane of the beam, whereby the major axis of the beam is deflected through a curving path with the electrons toward the outer edge of the curving beam having a longer path length in the field relative to those electrons toward the inner edge of the curving beam.

2. An electron beam gun assembly according to claim 1 wherein the magnetic field is of substantially uniform strength between said pole pieces.

3. An electron beam gun assembly according to claim 1 wherein the magnetic field between said pole pieces is of decreasing strength transversely of the initial path in the general direction toward which the electron beam is deflected.

4. An electron beam gun assembly according to claim 1 wherein the flux lines in the fringe regions of said transverse rmagnetic field are bowed for effecting lateral focusing and the beam producing means is disposed within one of the fringe regions.

5. An electron beam gun assembly according to claim 1 wherein said pole pieces have generally parallel sides and the opposed faces of said pole pieces are generally parallel.

6. An electron beam gun assembly according to claim 5 wherein a magnet extends between said pole pieces at one end thereof.

7. An electron beam gun assembly according to claim 5 wherein a magnet extends between said pole pieces at each end thereof, said magnets being identically oriented.

8. An electron beam gun assembly according to claim 5 wherein said producing and directing means include an elongated emitter, and wherein said pole pieces are generally parallel with said emitter.

9.. An electron beam gun assembly according to claim 1 wherein said pole pieces are diverged outwardly with respect to each other opening generally in the direction toward which the electron beam is deflected.

10. An electron beam gun assembly in accordance with claim 1 in which the beam producing means includes a filament disposed parallel to said transverse plane, a D0. power supply means is provided for passing D.C. current through said filament, and the ratio between the spacing of the pole pieces from each other and from said filament is of the order of 4 or 5 to 3.

11. An electron beam gun assembly in accordance with claim 7 in which the beam producing means includes a filament disposed parallel to said transverse plane, a DC. power supply means is provided for passing DC. current through said filament, the ratio between the spacing of the pole pieces from each other and from said filament is of the order of 4 or 5 to 3, the pole pieces are disposed rearwardly of the surface of the target, the beam producing means is disposed rearwardly of the surface of the target, and the major axis of the beam is directed rearwardly of the surface of the target.

12. An electron beam gun assembly in accordance with claim 3 in which the beam producing means includes a filament disposed parallel to said transverse plane, a DC. power supply means is provided for passing a DC. current through said filament, and the ratio between the spacing of the pole pieces from each other and from said filament is of the order of 4 or 5 to 3.

References Cited JAMES W. LAWRENCE, Primary Examiner V. LA FRANCHI, Assistant Examiner US. Cl. X.R. 

